In conventional manner, such apparatus comprises an emission laser and a receiver that includes an edge filter together with means for processing on the basis of the power P.sub.T transmitted through the filter. One particular known use lies in incoherent Doppler wind Lidar (IDWL) for detecting wind.
Incoherent Lidar type systems suffer from technical problems associated with:
the low level of power received which is due specifically to the size of the receive telescope, with the upper limit on said size being determined by considerations of mass and expense, and correspondingly by effective use of the power received; PA1 the alignment constraints on the numerous solid components as required by conventional systems; and PA1 constraints on the mechanical stability of such equipment, in particular when on board a satellite. PA1 the effect of the return signal being subject to dispersion in the atmosphere, known as "speckle"; and PA1 the various spectral characteristics of backscattering by aerosols and by molecules.
In the particular case of an IDWL type Lidar, two additional problems can arise:
IDWL type Lidar systems measure the frequency displacement to which laser radiation backscattered by the atmosphere is subjected, and it does so by means of frequency-selective means which process the backscattered optical signal prior to detection. The differences between the various IDWL type Lidar systems lie essentially in the nature of the frequency-selective means.
It is known to use a frequency discriminator that associates a Fabry-Perot etalon (FPE) and an edge filter. Such a system is described in particular: in the article by C. L. Korb, B. M. Gentry, and C. Y. Weng, entitled "Edge technique: theory and application to Lidar measurement of atmospheric wind", published in Applied Optics No. 31, 1992, pp. 4202-4213; in U.S. Pat. No. 5,216,477 (Korb); and in the article by B. M. Gentry and C. L. Korb entitled "Edge technique for high-accuracy Doppler velocimetry", published in Applied Optics No. 33, 1994, pp. 5770-5777.
In that edge filter technique or "edge technique", a shift in the frequency of the backscattered laser radiation is converted into a variation in the amplitude of the light that passes through the Fabry-Perot interferometer.
That technique suffers from the drawback of using only a portion of the backscattered power that it receives, and in addition, another portion of said received power is used for normalizing the signal. Unfortunately, as mentioned above, the power received is limited by the size of the receiving telescope.
It has been suggested that this power can be increased by replacing a single telescope which is heavy and expensive with a plurality of smaller telescopes that are lighter and less expensive. Reference can be made in particular to the article by S. Ishii et al., entitled "Optical fiber coupled multi-telescope Lidar system: application for a Rayleigh Lidar", published in "Review of Scientific Instruments", No. 67, 1996, pp. 3270-3273. However, the amount of power received remains quite low and the problem of optimizing use of the received power remains and no solution is found for it.
In known systems which incorporate an edge filter, it will be observed that the photodetection devices implement PIN diodes, avalanche photodiodes (APD), or indeed photomultiplier tubes.